Various forms of propulsion have been used to propel marine vessels over or through the water. One type of propulsion system comprises a prime mover, such as an engine or a turbine, which converts energy into a rotation that is transferred to one or more propellers having blades in contact with the surrounding water. The rotational energy in a propeller is transferred by contoured surfaces of the propeller blades into a force or “thrust” which propels the marine vessel. As the propeller blades push water in one direction, thrust and vessel motion are generated in the opposite direction. Many shapes and geometries for propeller-type propulsion systems are known.
Other marine vessel propulsion systems utilize waterjet propulsion to achieve similar results. Such devices include a pump, a water inlet or suction port and an exit or discharge port, which generate a waterjet stream that propels the marine vessel. The waterjet stream may be deflected using a “deflector” to provide marine vessel control by redirecting some waterjet stream thrust in a suitable direction and in a suitable amount.
A requirement for safe and useful operation of marine vessels is the ability to steer the vessel from side to side. Some systems, commonly used with propeller-driven vessels, employ “rudders” for this purpose. Other systems for steering marine vessels, commonly used in waterjet-propelled vessels, rotate the exit or discharge nozzle of the waterjet stream from one side to another. Such a nozzle is sometimes referred to as a “steering nozzle.” Hydraulic actuators may be used to rotate an articulated steering nozzle so that the aft end of the marine vessel experiences a sideways thrust in addition to any forward or backing force of the waterjet stream. The reaction of the marine vessel to the side-to-side movement of the steering nozzle will be in accordance with the laws of motion and conservation of momentum principles, and will depend on the dynamics of the marine vessel design.
It is understood that while particular control surfaces are primarily designed to provide force or motion in a particular direction, these surfaces often also provide forces in other directions as well. Nonetheless, those skilled in the art appreciate that certain control surfaces and control and steering devices have a primary purpose to develop force or thrust along a particular axis. For example, in the case of a reversing deflector, it is the backing direction in which thrust is provided. Similarly, a rudder is intended to develop force at the stern portion of the vessel primarily in a side-to-side or athwart ships direction, even if collateral forces are developed in other directions. Thus, net force imparted to a marine vessel should be viewed as a vector sum process, where net or resultant force is generally the goal, and other smaller components thereof may be generated in other directions at the same time.
As noted above, a class of marine craft is propelled by multiple steerable propeller drives. FIGS. 1A-1C illustrate various views of a stern/out drive that can be used in combination and FIGS. 1D-1E illustrate various views of a surface drive 111 that can be used in combination as outboard motors. As these terms may be used interchangeably herein, the use of one term shall not imply that the scope of this disclosure is limited to one specific type of drive. The scope of this disclosure includes twin-drive systems, as well as systems comprising more than two drives. A quad-arrangement employing four drives, wherein a pair of drives is installed on each of two hulls of a catamaran hull form, is but one example of a system that can benefit from this disclosure.
A notional single-drive system is depicted in FIGS. 2A-2B, and a notional twin-drive system is shown in FIGS. 2C-2D. The twin-drive system illustrated in FIGS. 2C-2D comprise a port stern drive 205 and starboard stern drive 206 and a mechanical link known as a tie-bar 207. The primary purpose of the tie bar 207 is to prevent the closely-spaced drives 205, 206 from colliding into each other in order to avoid damage to the craft or injury or death to persons onboard.
Referring to FIGS. 3A-3B, in systems employing surface drives or ventilating propellers, the propellers 310, 311, 314 and 315 can be partially submerged for varying amounts of time, during which time the propellers can develop substantial lateral (athwartships) and vertical forces. In multiple-drive installations of this kind, the rotation of the at least two of the propellers typically opposes each other. When a tie bar is used in these installations, a substantial net force is exerted on the tie-bar due to the substantially equal and opposite lateral forces generated by the propellers. For example, as shown in FIG. 3A, tie bar 312 undergoes outward tension 313 when the propellers 310, 311 are outboard rotating; also as shown in FIG. 3B, tie bar 316 undergoes compression forces 317 if the propellers 314, 315 are inboard rotating. By virtue of the tie-bar connection, the lateral forces are substantially cancelled out and the steering drives are not subjected to any significant load associated with the lateral force component of the partially submerged propellers.
In view of the above discussion, and in view of other considerations relating to design and operation of marine vessels, it is desirable to have a marine vessel control system which can provide thrust forces in a plurality of directions, and which can control thrust forces in a safe and efficient manner.